Dolphins (海豚) live their whole lives in water. But they are mamma& (哺乳动物J) that live in water. That’ s why a dolphin comes to the top of the water so often. A dolphin’ s nose isn’ t easy to see. It’ s just a tiny hole on top of the dolphin’ s head. This hole is called a blowhole. A dolphin comes up every half minute or so to breathe. It opens its blowhole and takes in air. Then it closes its blowhole tight and dives into the water again. But a dolphin can stay under the water for six or seven minutes if it has to. It holds its breath.
A dolphin hears with two ears. It hears all sorts of small noises under water. It hears small stones rolling. It can hear sounds fish and other dolphins make. A dolphin’s ears may be extra small, but its mouth is extra big. It has to be big. It holds almost one hundred teeth. These teeth are all the same shape and size. They are used for catching food, but not for chewing (咀嚼) it. A dolphin opens its big mouth. It seizes (抓住) a fish firmly between its two rows of teeth. Then it swallows (吞下) the whole fish.
A dolphin swims very fast. It is said that a dolphin swims faster than anything else in the water. So engineers are studying its skin and shape. They are learning ways to make boats go faster through the water.
It is well known that dolphins are very smart. Some scientists say they are smarter than dogs. Others say they are smarter than apes (类人猿).
The word "smart" in the last paragraph means ______.